Hereafter, conventional art relating to the present invention will be explained by exemplifying growing of a silicon single crystal.
An apparatus for growing a single crystal used for producing a silicon single crystal by the CZ method generally comprises a crucible containing a raw material melt, which can be moved upwardly and downwardly, and a heater disposed so as to surround the crucible, both of which are provided in a main chamber for growing a single crystal, and a pulling chamber for containing and taking out a grown single crystal is continuously provided above the main chamber. When a single crystal is produced by using such an apparatus for growing a single crystal, a seed crystal is immersed into the raw material melt and carefully pulled upwardly with rotation to grow a rod-like single crystal, while the crucible is moved upwardly according to the growth of the crystal so that the melt surface should be always maintained at a constant height in order to obtain desired crystal quality.
Further, when the single crystal is grown, the seed crystal attached to a seed holder is immersed into the raw material melt, and then the seed crystal is pulled upwardly with rotation in a desired direction by carefully winding up a wire by means of a pulling mechanism to grow a single crystal ingot at the end of the seed crystal. In this case, in order to eliminate dislocations generated when the seed crystal is brought into contact with the melt, the crystal in an early stage of the growth is once made thin to a small diameter of about 3 to 5 mm, and then the diameter is increased after the dislocation are eliminated so as to grow a single crystal ingot of desired quality.
At this time, the pulling rate for a portion having a constant diameter of the single crystal ingot is usually extremely slow, i.e., about 0.5 to 1 mm/min, and if it is pulled fast by constraint, there are arisen various problems. For example, the growing single crystal is deformed and thus a cylindrical product having a constant diameter can no longer be obtained, slip dislocations are generated in the single crystal ingot, the crystal is detached from the melt and thus it cannot be a product and so forth. Therefore, increase of the crystal growth rate is limited.
However, for the purpose of improving productivity and reducing cost in the production of single crystal ingots by the aforementioned CZ method, increase of the single crystal growth rate is the most effective means, and various improvement have hitherto been made to achieve increase of the single crystal growth rate.
The pulling rate, i.e., the single crystal growth rate, is determined by the heat balance of the growing crystal. The heat quantity incorporated into the crystal consists of inflow heat quantity from the melt and the heater and solidification latent heat generated when the melt crystallizes. When the heat balance of the growing crystal is considered, it is necessary that outflow heat quantity emitted out of the crystal from the crystal surface and through the seed crystal should be equal to the sum of the inflow heat quantity and the solidification latent heat. The solidification latent heat depends on the volume of the crystal growing per unit time. Therefore, in order to increase the crystal growth rate, it is necessary to compensate increase of solidification latent heat provided by increase of the crystal growth rate by reducing the inflow heat quantity or increasing the outflow heat quantity.
Therefore, it is generally used a method of efficiently removing heat emitted from the crystal surface to increase the outflow heat quantity.
As one of such means, there were proposed apparatuses in which the pulling rate is increased by providing cooling means in the main chamber so as to surround a single crystal ingot under pulling and thereby efficiently cooling the single crystal ingot under pulling. For example, there is the apparatus disclosed in Japanese Patent Laid-open (Kokai) Publication No. 6-211589. In this apparatus, a gas flow guide cooling cylinder having a double structure consisting of an outer cooling cylinder composed of metal and an inner cooling cylinder composed of graphite or the like is provided from the bottom portion of the pulling chamber to the inside of the main chamber so as to concentrically surround a single crystal ingot under pulling and thereby heat generated in the inner cooling cylinder is transferred to the outside by the outer cooling cylinder, so that temperature increase of the inner cooling cylinder should be suppressed and cooling efficiency of the crystal should be improved.
Apparatuses utilizing a cooling medium such as water in order to more efficiently cool a growing single crystal are also disclosed. For example, in the apparatus for growing a single crystal disclosed in Japanese Patent Laid-open (Kokai) Publication No. 8-239291, a cooling duct for circulating a liquid refrigerant is provided in a main chamber and a cooling member composed of a material having high heat conductivity such as silver is provided below the duct so as to rapidly transfer heat emitted from crystal surface to the outside and thereby attain effective cooling of the crystal. However, if fluid such as water generally used as the cooling medium become close to the melt surface heated to a high temperature exceeding 1000° C., it may be a cause of phreatic explosion and thus dangerous. Therefore, in this apparatus, safety is secured by separating the cooling duct from the melt surface.
However, in these days, the diameter of silicon wafers became larger and, in particular, mass production of silicon wafers having a diameter of 300 mm is contemplated. Thus, it has become clear that cooling of the crystals become insufficient even when such cooling structures as mentioned above are used and thus it is difficult to use further higher crystal growth rate, so that decrease of productivity of crystals is invited.
That is, it was found that there was a problem as described below. That is, such a cooling structure as disclosed in Japanese Patent Laid-open (Kokai) Publication No. 8-239291 can provide a certain degree of cooling effect, because a cooling member having high heat conductivity can be brought close to the melt surface of silicon. However, a restriction is imposed on the apparatus that a cooling duct for circulating a liquid refrigerant for cooling the cooling member must be sufficiently separated from the melt surface so that it should not be brought into contact with the melt surface when the crucible containing the melt is elevated to the maximum height that the crucible can reach, due to the problem concerning the safety described above. Therefore, there is caused a problem that the cooling effect provided by the cooling medium is unlikely to reach the end of the cooling member.